1. Field of the Invention
The present invention relates to an apparatus for exposing a substrate like a semiconductor wafer to X-rays in the lithographic process in the manufacture of semiconductor devices, and more particularly, to an X-ray transmitting window which forms a partitioning wall of two vessels having a relative pressure difference and which transmits X-rays so as to allow X-rays to be transferred between the vessels.
2. Description of the Related Art
There is an increasing demand for fine circuit patterns in order to provide high-density and high-performance semiconductor integrated circuits. The possibility of formation of fine patterns on the order of 0.1 .mu.m or less is the primary reason for developing X-ray lithography. A promising high-output X-ray source employed in such an X-ray lithography is SOR (synchrotron orbital radiation). In X-ray lithography which employs SOR, X-rays generated under a high vacuum of about 10.sup.-10 Torr are irradiated onto a sample, such as a semiconductor wafer, generally placed in an atmosphere having 1 atm. To achieve this, an X-ray transmitting window made of a material having a high X-ray transmittance, such as beryllium (Be), is provided between the SOR source and the sample chamber.
For the reason mentioned above, a pressure difference substantially equal to 1 atm is applied to the X-ray transmitting window. In the case of an X-ray transmitting window made of a Be plate and having an opening area of, for example, 30 mm.times.30 mm, such a pressure difference can be resisted if the thickness is about 0.2 mm. However, such a Be X-ray transmitting window has the following drawbacks.
(1) 10 .ANG. X-rays which pass through Be are attenuated at about 0.2 dB/.mu.m. This attenuation rate increases as the wavelength increases. Thus, the use of X-rays of short wavelengths in x-ray lithography is advantageous from the viewpoint of transmittance. However, X-ray absorber (which may be a gold film) on the exposure mask readily transmits X-rays of short wavelengths. Consequently, in X-ray lithography, the contrast of the X-ray absorber patterns is reduced, making exposure of fine patterns difficult. Thus, the use of a Be X-ray transmitting window which is as thin as possible and of X-rays of long wavelengths is desired. To achieve these objectives, it is necessary for the thickness of the Be X-ray transmitting window to be reduced to 1/10th or less of that of the conventional window.
(2) To achieve a high throughput in X-ray lithography, it is desired to increase the opening area of the X-ray transmitting window to about several tens of millimeters x several tens of millimeters.
Currently, it is very difficult to provide a Be X-ray transmitting window which satisfies the aforementioned requirements.